1. Field of the Invention
This invention relates to ultra high strength steels with a yield stress of not less than 110 kgf/mm.sup.2 having excellent strength, toughness and stress corrosion cracking resistance in sea water.
2. Related Art Statement
There has recently been taken a great interest in deep sea such as submarine resources exploitation, submarine crust and geological surveys or the like. In the industry of shipbuilding, the development and construction of pressure structures such as vessel for deep sea and the like, which are related to the submarine exploitation, have been faced with interest, and therefore an effort has been made to develop building techniques inclusive of welding.
The offshore structures such as deep-sea vessel and so on must not be deformed and destroyed by pressure, and therefore the maintenance of safety is taken up as a most significant item. The material for use in these structures is required to have a high ratio of strength to weight, namely a high strength and an excellent toughness owing to the necessity of providing a spherical shell having a high structure efficiency. Additionally, in case of using these high strength materials in atmosphere and environment different from air, investigations should particularly and sufficiently be made on the stress corrosion cracking.
In order to respond to such a strong demand for materials with more safety and higher reliability, the development and quality improvement of Ni-containing low alloy steel as an ultra high strength steel have been performed. For instance, there are developed many methods of producing these steels, such as Ni-Cr-Mo-V series high strength and high toughness steel with a yield stress of not less than 100 kgf/mm.sup.2 characterized by satisfying C+1/8Mo+V&gt;0.26 and Cr&lt;0.8Mo as disclosed in Japanese Patent laid open No. 56-9,358, an Ni-Cr-Mo-V series ultra high strength steel with a yield stress of not less than 110 kgf/mm.sup.2 wherein high strength and toughness are obtained at a wide cooling rate in the hardening treatment as disclosed in Japanese Patent laid open No. 57-188,655, a high toughness hardened and tempered-type Ni-containing steel treated for extremely low phosphorus and extremely low sulfur, and the like.
These methods are effective to increase the toughness. However, in consideration of service environments, it is hard to say that the resulting steels are sufficiently safe in use because no investigation is made considering the stress corrosion, for example, in sea water.
With respect to the stress corrosion cracking of the ultra high strength steel, the theory of linear fracture dynamics by B. F. Brown in U.S. NRL is accepted, whereby there is adopted a method of quantitatively determining what fracture behavior a material having some defects exhibits in corrosion environment by using a K-value at the top of the crack.
That is, a constant stress test is made with respect to a notched specimen at various levels of K-value under service environment by facilitating the occurrence of delayed fracture at the top of notch under extremely severe conditions, from which a critical value, K.sub.ISCC value for producing no fracture below a certain K-value is measured to a evaluate a stress corrosion cracking resistance.